1. Field of the Invention
The present invention relates to a process for producing a ceramic substrate, and more particularly, to a process for producing a ceramic substrate to which a non-shrinkage process is applied.
2. Description of the Related Art
In order to implement a method for producing a ceramic substrate according to a non-shrinkage process, an unfired composite laminate is produced and includes an unfired base material layer including, as its main component, a low-temperature sintering ceramic material, and a shrinkage suppression layer including, as its main component, a hard sinterable ceramic powder which is not substantially sintered at the sintering temperature of the low-temperature sintering ceramic material, in which the shrinkage suppression layer is provided on at least one principal surface of the base material layer.
Then, the unfired composite laminate is fired at the sintering temperature of the low-temperature sintering ceramic material, thereby providing a sintered ceramic substrate of the base material layer. In this firing step, the hard sinterable ceramic powder included in the shrinkage suppression layer is not substantially sintered, and thus, the shrinkage suppression layer does not substantially shrink. For this reason, the shrinkage suppression layer restrains the base material layer, thereby substantially shrinking the base material layer only in the thickness direction, while shrinkage in the principal surface direction is suppressed. As a result, non-uniform deformations are less likely to occur in the obtained ceramic substrate, and the accuracy of the shape and size in the planar direction of the ceramic substrate can be improved.
Next, the shrinkage suppression layer described above is removed, thereby extracting a ceramic substrate. It is to be noted that when a multilayer ceramic substrate is to be produced as the ceramic substrate, the unfired composite laminate described above includes a plurality of stacked base material layers.
In the method for producing a ceramic substrate according to the non-shrinkage process described above, the shrinkage suppression layer is required to have outstanding restraining performance in the step of firing and have excellent removal performance after the step of firing. Techniques relating to the removal performance of such a shrinkage suppression layer include, for example, a technique described in Japanese Patent Application Laid-Open No. 7-30253.
While the technique described in Japanese Patent Application Laid-Open No. 7-30253 is directly related to the removal performance of the shrinkage suppression layer, the technique promotes the removal of an organic binder in a base material layer in the process of firing, such that the removal of the organic binder in the base material layer via through holes is facilitated by applying a hole forming process to the shrinkage suppression layer, filling the numerous formed through holes with a resin paste having a decomposition temperature lower than the organic binder included in the base material layer, and thermally decomposing the resin in the through holes first in the step of firing. According to this technique described in Japanese Patent Application Laid-Open No. 7-30253, the effect of facilitating removal of the shrinkage suppression layer can also be expected because the numerous through holes are provided in the shrinkage suppression layer after the step of firing.
However, when the technique described in Japanese Patent Application Laid-Open No. 7-30253 is applied to try to form a shrinkage suppression layer which is easily removed after firing, the number of through holes formed in accordance with a hole forming process and the cross-sectional area of the through holes will be increased, which will lead to a decrease in the restraining performance provided by the shrinkage suppression layer.
In order to improve the restraining performance, the reactivity of the shrinkage suppression layer with the base material layer is preferably increased, and the rigidity of the shrinkage suppression layer itself is preferably increased. However, when the reactivity is increased and when the rigidity is also increased, the shrinkage suppression layer is not easily fractured in the step of removing the shrinkage suppression layer, and it is also difficult to peel the shrinkage suppression layer from the ceramic substrate, thus resulting in a decrease in removal performance. As described above, it is difficult for the shrinkage suppression layer to achieve a balance between restraining performance and removal performance.
Furthermore, when the technique described in Japanese Patent Application Laid-Open No. 7-30253 is used to try to improve the removal performance of the shrinkage suppression layer, it will be necessary to apply a hole forming process to the shrinkage suppression and fill the formed through holes with a resin paste, thus causing the production efficiency to be decreased.